In the northern Midlands, the annual evaporation is nearly 1,500 mm due largely to the prevalence of winds coming from the Western Tiers, which become warmer and drier as they descend. This area of high evaporation extends to the lower Derwent Valley and the Huon. Mean monthly evaporation is nearly 200 mm in the summer in these areas, but is closer to 30 mm in winter. Evaporation in western, central and southern regions is much lower, normally less than 750 mm each year, ranging between 15 mm per month in winter and about 100 mm per month in summer.